MOSFET refers to a metal oxide semiconductor field effect transistor or any other similar semiconductor switch having very high input impedance as compared to bi-polar transistors. The power transistor has a control terminal which, in the case of the bi-polar transistor, is referred to as the base and in the case of the MOSFET, is referred to as the gate.
Field-controlled switches, including power MOSFETs and IGBTs include a capacitance known as the Miller capacitance C.sub.M between the drain and gate and a second capacitance C.sub.1 between the gate and source. To turn on a field-controlled switch, the gate-source capacitor C.sub.1 must be charged and the Miller capacitor C.sub.M discharged. To turn off the field-controlled switch, a discharge of the gate-source capacitor C.sub.1 is necessary and the Miller capacitor must be charged. A field-controlled switch can be supplied with voltage from a driver which receives both a power signal and a switching signal in the form of a combined switching power signal. This switching power signal consists of a positive pulse train to the driver. During the off-state, the driver leaks energy. This energy is needed to power the driver to put it in a state where it is capable of providing a combined an ON/OFF command signal to the field-controlled switches, for example, in an inverter leg. The off-state leakage currents discharge the driver. This causes the operation of active components (transistors and ICs) to be weak and unstable; that is, the active components produce less current and voltage and also switch state without being commanded to do so. Presently, the first pulse in the pulse train is used to restore power to the driver which was lost through leakage.
One result of the weak and unstable state is spurious transitions. Power MOSFETs in their non-conducting condition are subject to a spurious turn-on if the drain-source voltage changes with a high rate of change. Also, a power MOSFET switch presently conducting may spuriously turn off if its gate charge is permitted to discharge during its conducting interval. These spurious transitions are subject to both external circuit conditions and to parasitic elements of the MOSFET. While an unwanted transition of the MOSFET from the on-state to the off-state may be damaging to the performance of an overall power system, a spurious transition from the off-state to on-state is frequently severely damaging to the MOSFET and may cause its destruction. One solution to alleviate this problem is disclosed in U.S. Pat. No. 4,748,351, "Power MOSFET Gate Driver Circuit" by Barzegar. Barzegar teaches utilizing a gate drive circuit to control conductivity of a power MOSFET. There the drive isolates the gate from noise signals and provides an initial positive bias to turn on the power MOSFET into conduction until a specific turn off bias signal is applied. However, the positive bias is produced with the aid of an additional control MOSFET. While, the solution prevents spurious turn operation of the power MOSFET, it does so by adding as a solving component the vary element that is susceptible to spurious operation--a MOSFET.
Field-controlled switches need to have a stable power supply and also need to be kept off during a dead time if the switch is to be used in an inverter. To provide the dead time (the time after the turning off of one switch before the turning on of the complementary switch, during which neither switch is on) for the inverter, separate circuitry outside the driver is required, as shown in U.S. Pat. No. 4,554,512 "Switching Amplifier with MOSFET Driver Circuit".